Data storage device reverse biasing head element to counter electro-migration

ABSTRACT

A data storage device is disclosed comprising a first head actuated over a first disk surface, the first head comprising a plurality of elements including a first element. During a first write operation of the first head, a first bias signal having a first polarity is applied to the first element, and a write interval of the first write operation is measured. During a non-write mode of the first head, a second bias signal having a second polarity opposite the first polarity is applied to the first element during a reverse bias interval that is based on the write interval of the first write operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/360,902, filed on Mar. 21, 2019, which application claims benefit ofU.S. Provisional Patent Application Ser. No. 62/646,849, filed Mar. 22,2018, both of which are herein incorporated by reference.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

Embodiments of the present disclosure generally relate to a data storagedevice.

Description of the Related Art

Data storage devices such as disk drives comprise a disk and a headconnected to a distal end of an actuator arm which is rotated about apivot by a voice coil motor (VCM) to position the head radially over thedisk. The disk comprises a plurality of radially spaced, concentrictracks for recording user data sectors and servo sectors. The servosectors comprise head positioning information (e.g., a track address)which is read by the head and processed by a servo control system tocontrol the actuator arm as it seeks from track to track.

Data is typically written to the disk by modulating a write current inan inductive coil (write coil) to record magnetic transitions onto thedisk surface in a process referred to as saturation recording. Duringread-back, the magnetic transitions are sensed by a read element (e.g.,a magneto-resistive element) and the resulting read signal demodulatedby a suitable read channel. Heat assisted magnetic recording (HAMR) is arecent development that improves the quality of written data by heatingthe disk surface during write operations in order to decrease thecoercivity of the magnetic medium, thereby enabling the magnetic fieldgenerated by the write coil to more readily magnetize the disk surface.Microwave assisted magnetic recording (MAMR) is also a recentdevelopment that improves the quality of written data by using a spintorque oscillator (STO) to apply a high frequency auxiliary magneticfield to the media close to the resonant frequency of the magneticgrains, thereby enabling the magnetic field generated by the write coilto more readily magnetize the disk surface. Since the quality of thewrite/read signal depends on the fly height of the head, conventionalheads may also comprise an actuator for controlling the fly height. Anysuitable fly height actuator (FHA) may be employed, such as a heaterwhich controls fly height through thermal expansion, or a piezoelectric(PZT) actuator. A data storage device may also employ dual FHAs toachieve a first fly height during write operations and a second,different fly height during read operations.

SUMMARY OF THE DISCLOSURE

A data storage device is disclosed comprising a first head actuated overa first disk surface, the first head comprising a plurality of elementsincluding a first element. During a first write operation of the firsthead, a first bias signal having a first polarity is applied to thefirst element, and a write interval of the first write operation ismeasured. During a non-write mode of the first head, a second biassignal having a second polarity opposite the first polarity is appliedto the first element during a reverse bias interval that is based on thewrite interval of the first write operation.

In one embodiment, a data storage device comprises: a first disksurface; a first head actuated over the first disk surface, the firsthead comprising a plurality of elements including a first element; andcontrol circuitry configured to: during a first write operation of thefirst head, apply a first bias signal having a first polarity to thefirst element; measure a write interval of the first write operation;and during a non-write mode of the first head, apply a second biassignal having a second polarity opposite the first polarity to the firstelement during a reverse bias interval that is based on the writeinterval of the first write operation.

In another embodiment, a method of operating a data storage devicecomprises: during a first write operation of a first head to a firstdisk surface, applying a first bias signal having a first polarity to afirst element of the first head; measuring a write interval of the firstwrite operation; and during a non-write mode of the first head, applyinga second bias signal having a second polarity opposite the firstpolarity to the first element during a reverse bias interval that isbased on the write interval of the first write operation.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this disclosure and are therefore not to beconsidered limiting of its scope, for the disclosure may admit to otherequally effective embodiments.

FIG. 1A shows a data storage device in the form of a disk driveaccording to an embodiment comprising a head actuated over a disk.

FIG. 1B shows a head according to an embodiment comprising at least oneelement that is biased with a first bias signal during a writeoperation, and biased with a second, opposite polarity bias signalduring a non-write operation in order to counter an electro-migrationeffect.

FIG. 1C is a flow diagram according to an embodiment wherein the reversebias signal (opposite polarity bias signal) is applied to a writeelement of the head during a reverse-bias interval that is based on thewrite interval of write operation (s).

FIG. 2A shows an embodiment wherein an amplitude of the reverse biassignal equals the amplitude of the write bias signal, and the reversebias interval equals the write interval of a write operation.

FIG. 2B shows an embodiment wherein an amplitude of the reverse biassignal is less than the amplitude of the write bias signal, andtherefore the reverse bias interval is greater than the write intervalof a write operation (e.g., half the amplitude and twice the writeinterval).

FIG. 3A shows an embodiment wherein the disk drive comprises multipledisk surfaces and a corresponding head actuated over each disk surface.

FIG. 3B is a flow diagram according to an embodiment wherein the writeintervals for each head is accumulated, and during a write operationusing a first one of the heads, the reverse bias signal is applied to asecond one of the heads having the largest accumulated write intervals.

FIG. 4 shows an embodiment wherein a square wave reverse bias signal isapplied to at least two head elements during the reverse bias interval,wherein the square waves are phase offset and de-multiplexed onto therespective head elements.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DETAILED DESCRIPTION

In the following, reference is made to embodiments of the disclosure.However, it should be understood that the disclosure is not limited tospecific described embodiments. Instead, any combination of thefollowing features and elements, whether related to differentembodiments or not, is contemplated to implement and practice thedisclosure. Furthermore, although embodiments of the disclosure mayachieve advantages over other possible solutions and/or over the priorart, whether or not a particular advantage is achieved by a givenembodiment is not limiting of the disclosure. Thus, the followingaspects, features, embodiments and advantages are merely illustrativeand are not considered elements or limitations of the appended claimsexcept where explicitly recited in a claim(s). Likewise, reference to“the disclosure” shall not be construed as a generalization of anyinventive subject matter disclosed herein and shall not be considered tobe an element or limitation of the appended claims except whereexplicitly recited in a claim(s).

FIG. 1A shows a data storage device in the form of a disk driveaccording to an embodiment comprising a first head 2 ₀ actuated over thefirst disk surface 4 ₀, the first head 2 ₀ comprising a plurality ofelements including a first element 6 ₀ (FIG. 1B). The disk drive furthercomprises control circuitry 8 configured to execute the flow diagram ofFIG. 1C, wherein during a first write operation of the first head, afirst bias signal having a first polarity is applied to the firstelement (block 10). A write interval of the first write operation ismeasured (block 12), and during a non-write mode of the first head(block 14), a second bias signal having a second polarity opposite thefirst polarity is applied to the first element during a reverse biasinterval that is based on the write interval of the first writeoperation (block 16).

The first head shown in FIG. 1A may comprise any suitable elements, suchas a write coil, a heat assisted element for use in heat assistedmagnetic recording (HAMR), a fly height actuator, or a spin torqueoscillator (STO) or other suitable resistive element proximate the writeelement for use in microwave assisted magnetic recording (MAMR). Duringa write operation, a suitable bias signal (e.g., a positive bias currentin the embodiment shown in FIG. 1B) is applied to at least one of theelements, for example, to cause a STO to begin oscillating. In oneembodiment, the bias signal applied to the head element during writeoperation(s) results in an electro-migration effect (the transport ofmaterial caused by the gradual movement of ions in a conductor due tothe momentum transfer between conducting electrons and diffusing metalatoms). In one embodiment, in order to counter the electro-migrationeffect due to write operation(s), a bias signal having an oppositepolarity (e.g., a negative bias current as shown in FIG. 1B) is appliedto the head element during a non-write mode over a reverse bias intervalthat is based on the write interval(s) of the write operation(s).

FIG. 2A shows an embodiment wherein an amplitude of the reverse biassignal (opposite polarity bias signal) during the reverse bias intervalequals the amplitude of the write bias signal, and the reverse biasinterval equals the write interval of a write operation. In this manner,the electro-migration effect that occurs during the write operation iseffectively countered by the reverse bias signal. However, in oneembodiment applying a high amplitude reverse bias signal during thereverse bias interval may reduce the longevity of the head element due,for example, to increased heating of the head element. Accordingly inone embodiment shown in FIG. 2B, the amplitude of the reverse biassignal may be less than the amplitude of the bias signal applied duringthe write operation. In this embodiment, the reverse bias interval overwhich the reverse bias signal is applied is increased relative to thewrite interval of the write operation. In the example of FIG. 2B, theamplitude of the reverse bias signal is half the amplitude of the writebias signal applied during the write operation, and the reverse biasinterval for countering the electro-migration effect is twice the writeinterval. In other embodiments, the relationship between the amplitudeof the reverse bias signal and the length of the reverse bias intervalrelative to the amplitude of the write bias signal and the writeinterval may be configured based on any suitable function, such as anysuitable polynomial, exponential, etc.

The reverse bias signal may be applied to a head element during anysuitable non-write mode of the corresponding head. For example, in oneembodiment the reverse bias signal may be applied to the head elementduring an idle mode of the corresponding head. In yet anotherembodiment, the reverse bias signal may be applied during a readoperation of the corresponding head if reverse biasing the head elementwill not adversely affect the read operation. For example, in oneembodiment an STO may be reverse biased with an opposite polarity biassignal during a read operation since the STO will not oscillate when nowrite current is applied to the write coil.

FIG. 3A shows an embodiment wherein the disk drive comprises multipleheads 2 ₀-2 ₃ each actuated over a respective disk surface 4 ₀-4 ₃,wherein in one embodiment the write intervals for each head isaccumulated, and the reverse bias signal is applied during a non-writemode of the head based on the accumulated write intervals. An example ofthis embodiment is shown in the flow diagram of FIG. 3B wherein when awrite operation is executed (block 18) for one of the heads 2 ₀-2 ₃, thewrite bias signal is applied to an element of the head (block 20), andthe write interval of the write operation is accumulated for the head(block 22). Also in connection with executing the write operation, thehead having the largest accumulated write intervals which is not beingused for the write operation is selected (block 24). During the writeoperation, the reverse bias signal is applied to an element of theselected head (block 26), and the accumulated write intervals for thehead is decreased (block 28). In one embodiment, the amount theaccumulated write intervals is decreased at block 28 for the non-writehead is based on the amplitude of the reverse bias signal as describedabove with reference to FIGS. 2A and 2B.

In one embodiment, the control circuitry 8 of FIG. 1A is configured toapply the write bias signal to an element of the active head during awrite operation, and to concurrently apply the reverse bias signal to anelement of a selected one of the inactive heads during the writeoperation. That is, in one embodiment concurrently applying the writebias signal and the reverse bias signal to respective heads simplifiesthe design of the control circuitry 8, and may also minimize theinterfacing transmission lines between the control circuitry 8 and theheads. In an alternative embodiment, the reverse bias signal may beapplied to a selected head during a non-write mode, for example, duringa read operation or during an idle mode of the disk drive. In oneembodiment, the reverse bias signal may be applied during a readoperation to a head that is used to perform the read operation. Forexample, the reverse bias signal may be applied to an STO element of ahead while concurrently using the same head to read data from the disksince the STO will not oscillate when no write current is applied to thewrite coil.

In one embodiment, the head elements may be reversed biased over reversebias intervals so as to maintain a substantial equilibrium in theelectro-migration effect. In one embodiment, if the accumulated writeintervals for one of the heads increases above a predeterminedthreshold, write operations to the corresponding disk surface may beredirected to a different disk surface, or in another embodimentdeferred by temporarily caching the write data in a non-volatilesemiconductor memory (e.g., Flash memory). While the write operationsare redirected or deferred for a particular head, the head element(s)may be reverse biased to counter the electro-migration effect andthereby decrease the accumulated write intervals for the head. In oneembodiment, the threshold used to trigger a redirect or deferment ofwrite operations may include any suitable hysteresis before re-enablingwrite operations to the corresponding disk surface. That is, writeoperations to a particular disk surface may be redirected or deferreduntil the accumulated write intervals for the head falls below ahysteretic threshold.

In the embodiment of FIG. 1B, the write bias signal and reverse biassignal applied to the head element 6 ₀ is a bias current, for example,for biasing an STO of a MAMR disk drive. In other embodiments, the writebias signal and the reverse bias signal may be a bias voltage applied tothe head element 6 ₀ which causes a current to flow through the headelement. In addition, the reverse bias signal applied to the headelement during the reverse bias interval may be of any suitable form,such as a DC signal as shown in the embodiment of FIG. 2A. In anotherembodiment, the reverse bias signal may be applied as a periodic signalto the head element. For example, the reverse bias signal may be asquare wave having any suitable amplitude and duty cycle, wherein in oneembodiment a smaller duty cycle may increase the longevity of the headelement by decreasing stress (e.g., heating) of the head element.

FIG. 4 shows an embodiment wherein a square wave reverse bias signal isapplied to at least two head elements during the reverse bias interval,wherein the square waves are phase offset and de-multiplexed onto therespective head elements. For example, the reverse bias signal(generated as a DC signal) may be first applied to the element of head ifor a predetermined interval, and then applied to the element of head jfor a predetermined interval in an alternating sequence, therebyapplying a square wave reverse bias signal to each head element as shownin FIG. 4. Accordingly in this embodiment, the electro-migration effectof multiple head elements may be counteracted during the reverse biasinterval so as to minimize the overall time needed to equalize the headelements as well as prolonging the longevity of the head elements.

In other embodiments, a similar square wave reverse bias signal may beapplied to three or more head elements by de-multiplexing a DC biassignal over the multiple head elements during the reverse bias interval.In this embodiment, the duty cycle of the square wave reverse biassignal applied to each head element will be reduced proportional to thenumber of head elements being reverse biased. In one embodiment,reducing the duty cycle enables increasing the amplitude of the squarewave without stressing the head elements. In one embodiment, theamplitude and effective duty cycle of the reverse bias square wave maybe configured based on the accumulated write intervals for each headelement. For example, in one embodiment the larger the accumulated writeintervals, the larger the amplitude and/or duty cycle applied to thecorresponding head element.

Any suitable control circuitry may be employed to implement the flowdiagrams in the above embodiments, such as any suitable integratedcircuit or circuits. For example, the control circuitry may beimplemented within a read channel integrated circuit, or in a componentseparate from the read channel, such as a disk controller, or certainoperations described above may be performed by a read channel and othersby a disk controller. In one embodiment, the read channel and diskcontroller are implemented as separate integrated circuits, and in analternative embodiment they are fabricated into a single integratedcircuit or system on a chip (SOC). In addition, the control circuitrymay include a suitable preamp circuit implemented as a separateintegrated circuit, integrated into the read channel or disk controllercircuit, or integrated into a SOC.

In one embodiment, the control circuitry comprises a microprocessorexecuting instructions, the instructions being operable to cause themicroprocessor to perform the flow diagrams described herein. Theinstructions may be stored in any computer-readable medium. In oneembodiment, they may be stored on a non-volatile semiconductor memoryexternal to the microprocessor, or integrated with the microprocessor ina SOC. In another embodiment, the instructions are stored on the diskand read into a volatile semiconductor memory when the disk drive ispowered on. In yet another embodiment, the control circuitry comprisessuitable logic circuitry, such as state machine circuitry.

In various embodiments, a disk drive may include a magnetic disk drive,an optical disk drive, etc. In addition, while the above examplesconcern a disk drive, the various embodiments are not limited to a diskdrive and can be applied to other data storage devices and systems, suchas magnetic tape drives, hybrid drives (disk plus solid state), etc. Inaddition, some embodiments may include electronic devices such ascomputing devices, data server devices, media content storage devices,etc. that comprise the storage media and/or control circuitry asdescribed above.

The various features and processes described above may be usedindependently of one another, or may be combined in various ways. Allpossible combinations and subcombinations are intended to fall withinthe scope of this disclosure. In addition, certain method, event orprocess blocks may be omitted in some implementations. The methods andprocesses described herein are also not limited to any particularsequence, and the blocks or states relating thereto can be performed inother sequences that are appropriate. For example, described tasks orevents may be performed in an order other than that specificallydisclosed, or multiple may be combined in a single block or state. Theexample tasks or events may be performed in serial, in parallel, or insome other manner. Tasks or events may be added to or removed from thedisclosed example embodiments. The example systems and componentsdescribed herein may be configured differently than described. Forexample, elements may be added to, removed from, or rearranged comparedto the disclosed example embodiments.

While certain example embodiments have been described, these embodimentshave been presented by way of example only, and are not intended tolimit the scope of the inventions disclosed herein. Thus, nothing in theforegoing description is intended to imply that any particular feature,characteristic, step, module, or block is necessary or indispensable.Indeed, the novel methods and systems described herein may be embodiedin a variety of other forms; furthermore, various omissions,substitutions and changes in the form of the methods and systemsdescribed herein may be made without departing from the spirit of theembodiments disclosed herein.

While the foregoing is directed to embodiments of the presentdisclosure, other and further embodiments of the disclosure may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims that follow.

What is claimed is:
 1. A data storage device, comprising: a first disksurface; a first head actuated over the first disk surface, the firsthead comprising a plurality of elements including a first element; andcontrol circuitry configured to: during a non-write mode of the firsthead, apply a bias signal having a polarity to the first element duringa reverse bias interval that is based on a write interval of a writeoperation, wherein the polarity is opposite a polarity used in the writeoperation.
 2. The data storage device as recited in claim 1, wherein thefirst element is at least one of a write coil, a heat assisted element,a fly height actuator, a spin torque oscillator, and a resistiveelement.
 3. The data storage device as recited in claim 1, wherein thecontrol circuitry is further configured to: during the non-write mode ofthe first head, apply the bias signal to the first element during areverse bias interval that is based on accumulated write intervals. 4.The data storage device as recited in claim 1, wherein: the reverse biasinterval is different than the write interval by an amount that is basedon a difference between an amplitude of the bias signal and an amplitudeof a write signal.
 5. The data storage device as recited in claim 1,further comprising a second head actuated over a second disk surface,wherein the control circuitry is further configured to apply the biassignal to the first element during a write mode of the second head. 6.The data storage device as recited in claim 5, wherein the second headcomprises a second element, and the control circuitry is furtherconfigured to: during a non-write mode of the second head, apply thebias signal to the second element during a reverse bias interval that isbased on a write interval of a second write operation.
 7. The datastorage device as recited in claim 6, wherein the control circuitry isfurther configured to: during a non-write mode of the first and secondheads, apply the bias signal to one of the first and second elementsbased on accumulated write intervals.
 8. The data storage device asrecited in claim 7, wherein the control circuitry is further configuredto apply the bias signal to the first element when the accumulated writeintervals for the first head is longer than the accumulated writeintervals for the second head.
 9. The data storage device as recited inclaim 6, wherein the control circuitry is further configured to: whenaccumulated write intervals for the first head exceeds a threshold:temporarily disable write operations to the first disk surface; andapply the bias signal to the first element while the write operations tothe first disk surface are disabled.
 10. The data storage device asrecited in claim 9, wherein while the write operations to the first disksurface are disabled, the control circuitry is further configured toperform at least one of: redirect write operations initially targetingthe first disk surface to the second disk surface; and cache write dataof the write operations targeting the first disk surface.
 11. A methodof operating a data storage device, the method comprising: during anon-write mode of a first head having a first element, applying a biassignal having a polarity to the first element during a reverse biasinterval that is based on a write interval of a write operation.
 12. Themethod as recited in claim 11, wherein the first element is at least oneof a write coil, a heat assisted element, a fly height actuator, a spintorque oscillator, and a resistive element.
 13. The method as recited inclaim 11, further comprising: during the non-write mode of the firsthead, applying the bias signal to the first element during a reversebias interval that is based on accumulated write intervals.
 14. Themethod as recited in claim 11, wherein: the reverse bias interval isdifferent than the write interval by an amount that is based on adifference between a first amplitude of the bias signal and an amplitudeof a write signal.
 15. The method as recited in claim 11, furthercomprising applying the bias signal to the first element during a writemode of a second head to a second disk surface.
 16. The method asrecited in claim 15, further comprising: during a non-write mode of thesecond head having a second element, applying the bias signal to thesecond element during a reverse bias interval that is based on the writeinterval of a second write operation.
 17. The method as recited in claim16, further comprising: during a non-write mode of the first and secondheads, applying the bias signal to one of the first and second elementsbased on accumulated write intervals.
 18. The method as recited in claim17, further comprising applying the bias signal to the first elementwhen the accumulated write intervals for the first head is longer thanthe accumulated write intervals for the second head.
 19. The method asrecited in claim 16, further comprising: when accumulated writeintervals for the first head exceeds a threshold: temporarily disablingwrite operations to a first disk surface; and applying the bias signalto the first element while the write operations to the first disksurface are disabled.
 20. The method as recited in claim 19, whereinwhile the write operations to the first disk surface are disabled,further comprising at least one of: redirecting write operationsinitially targeting the first disk surface to the second disk surface;and caching write data of the write operations targeting the first disksurface.